The invention relates to an electric lamp comprising:
a glass lamp vessel closed in a gaslight manner and having a wall in which metal current conductors are enclosed, which conductors issue from the lamp vessel to the exterior; PA1 an electric element in the lamp vessel, electrically connected to the current conductors; PA1 a gas filling in the lamp vessel, the current conductors having a surface layer of metal phosphide.
Such an electric lamp is known from U.S. Pat. No. 3,798,058.
The known lamp is an electric incandescent lamp with a quartz glass lamp vessel. The current conductors are each built up from a metal foil which is enclosed in a pinch seal, and a metal wire connected thereto and issuing from the lamp to the exterior. An inner conductor made of molybdenum is fastened to the metal foil and connects the current conductor to the incandescent body.
Prior to lamp manufacture, the current conductors with the inner conductors fastened thereto are coated with tungsten phosphide or molybdenum phosphide by description.
The purpose of the coating is to protect the current conductors against oxidation during lamp manufacture, especially during heating of the lamp vessel while its seals are being made. Oxidation of the current conductors may lead to leaks in the lamp.
The coating also envisages to protect portions of the current conductors which are in contact with the atmosphere surrounding the lamp during lamp operation against oxidation. Since an oxide skin does not inhibit progressive oxidation and since the oxides are more voluminous than the metals, progressive oxidation of the current conductors leads to stresses in the lamp vessel, to chipping of glass and leaks in the lamp vessel, possibly resulting in its explosion.
The rate at which the current conductors are oxidized increases strongly with the temperatures which these conductors assume during operation. Lamp life accordingly decreases strongly with increasing operational temperature. According to IEC standard 357, therefore the maximum wall temperature of the lamp vessel at the area of the current conductor enclosed therein is 400.degree. C. and 450.degree. C. for a rated life of at most 300 and 15 hours, respectively.
The known lamp has an appearance which is different from that of a lamp whose current conductors are not coated. The current conductors are shiny and look as if they were made of silver where they are embedded in the glass of the lamp vessel. Non-coated current conductors on the other hand are dull and have a greyish colour.
The known lamp does offer protection against oxidation of the current conductors, but it has major disadvantages.
Considerable rejects occur during lamp manufacture because the metal foil is ruptured. These rejects generally amount to approximately 20% of the number of lamps manufactured. The rejects alone cause a considerable increase in the cost of the lamp.
In addition, the manufacture of the current conductors is time-consuming. The current conductors are, for example, exposed to the vapour of a tungsten phosphate or molybdenum phosphate melt, after which they are heated at a temperature of 700.degree.-1000.degree. C. for at least 3 hours in a reducing atmosphere. These operations result in a further increase in cost.